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UW research targets horn fly scourge variables

by Wyoming Livestock Roundup

Two cows, same University of Wyoming (UW) McGuire Ranch pasture, northeast of Laramie near Sybille Canyon. One cow has 383 horn flies sucking her blood. The other cow has four.

Why that remarkable difference?

Craig Calkins, a Cody High School graduate turned veterinarian, turned Army major and now a PhD student at the University of Wyoming, is helping UW Extension Range Specialist Derek Scasta unravel the mystery of a pestilence that costs the livestock industry billions of dollars in losses.

Calkins is eyeing whether shorter blood clotting times of individual animals clogs a fly’s attempts, whether a thicker hide frustrates flies and if elevation and environmental conditions, such as colder and wetter areas, affect fly parasitism.

Horn flies

Considered a filth fly, horn flies feast on a cow’s blood, leave to lay their eggs in manure, then fly back to their beef buffet. Eggs hatch after about two weeks, and a new generation begins.

The flies pierce the hide and inject an anti-coagulant to help free the flow of blood. Cattle swing their heads, slap their tails and twitch their skin in attempts to stop the biting.

“Seeing an animal with horn flies is a really discouraging situation when thinking about the animal,” said Scasta, who noted the constant irritation. “Cattle producers suffer production losses because the animal is losing blood but also because of these annoyance avoidance behaviors. Every time an animal picks its head up and swings, it’s not taking a bite of grass, so grazing time decreases.”

He said other scientists have found such cows produce less milk, which lowers weaning weights. The biting also slows growth rates of younger animals such as calves or yearlings.

Some animals will be bothered more than others in any group.

“They may all look similar as far as we can tell, but there will be some cows infested more than others,” Scasta said. “We’re trying to identify the traits those individual animals have that make them more or less susceptible to parasitism.”

Calkins’ study began with help from the military. The 438th Medical Detachment Veterinary Service Support from Fort Carson, Colo. helped draw blood from UW cattle, and Calkins analyzed blood-clotting times. 

“Hide thickness in three different areas was measured using ultrasound,” said Calkins.

“We were kind of surprised how fast some of the cows’ blood coagulated,” he added. “Sometimes, it would be clotted in the tube before we could even get to the machine to run it.”

Cattle studied include those in the Bighorn Mountains and lower altitude herds at the James C. Hageman Sustainable Agriculture Research and Extension Center near Lingle. Other cattle are near Cheyenne, Cody and on the McGuire Ranch.

Horn fly study

Calkins’ study requires documenting the extent of horn fly parasitism on cattle. He noted trends across the state show decreasing horn fly parasitism in the higher elevations.

“This is related to colder temperatures as elevation increases,” he said.

 “The sun illuminates the whole side of the cow, so the flies are really easy to see,” Calkins explained. “We’re counting flies all over the body. We stratify the head, side of the legs, the belly, brisket and tailhead.”

He noted only one side of a cow is counted.

“So realistically, whatever number we come up with is likely doubled,” he said.

Calkins noted the highest fly count was 383, noted on the Fourth of July last year, and that was just one side.

“The next highest counts were 319, 280, 229, 219, 205 and 190,” he said. “The lowest was four. So what’s the difference between the cows on the top and the cows at the bottom? That’s what I’m trying to find out.”

Study objective

The goal is to identify traits making an animal more prone to parasitism, so the researchers can guide ranchers culling decisions.

“Potentially removing outlier cows we know are super-prone to parasitism could save producers a lot of money,” said Calkins.

“Producers use various treatment options,” said Scasta. “These include a fed-through product containing an insect growth regulator (IGR), spraying, ear tags containing an insecticide and back rubbers that disperse a chemical.”

Scasta explains each has its limitations, including insects becoming resistant to the insecticides or the need for re-treatment.

The data from the study may help develop options to help producers save money.           

“If we are going to a have an integrated pest management approach, some of these other things will be really important so we can select for certain cows that are less susceptible,” Scasta said.

This article was written by Dr. Steve Miller and is courtesy of the University of Wyoming. For more information, visit or e-mail Miller at

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